The present invention generally relates to a method of detecting a physical object by employing a radio wave and an apparatus for carrying out the method. More particularly, the invention concerns an object detecting method and system suited for identifying a position or location, shape or configuration and material of an object buried in the earth, although application of the invention is not restricted thereto.
In connection with the detection of an object by using a radio wave, there is a method shown in Japanese Patent Application No. 247689/1983 filed Dec. 29, 1983 under the title "RADAR SYSTEM" (not yet laid open) and a corresponding U.S. application Ser. No. 687,862 filed on Dec. 31, 1984 under the title "RADAR APPARATUS" and a paper titled "UNDERGROUND RADAR SYSTEM" SANE 79-40, reported on Jan. 25, 1980, "Technical Report of IECE in Japan", which document is an elucidation of the technology described in the patent applications mentioned above. The block diagram of the system of the prior Japanese Patent Application is illustrated in FIG. 1 of the accompanying drawings.
Referring to FIG. 1, an oscillator 3 oscillates at a very high frequency, whose output energy is radiated toward the ground or earth from an antenna 2 through a transmission/reception selector 4 in the form of a pulse-shape radio wave. The echo signal from a buried target object 1 is received by the antenna 2 and supplied through the transmission/reception selector 4 to a display device 9 after having been amplified by an amplifier 5. A controller 10 memorizes a time point at which the radio wave was transmitted and compares it with a time point at which the echo signal was received, to produce a time difference signal on the basis of which the time taken for the traveling of radio wave is arithmetically determined. Further, the controller 10 operates to determine a position on the display device 9 at which the received signal is to be displayed in dependence on the aforementioned time difference or interval between the transmission of the radio wave and the reception of the echo, to therby display the buried object 1 on the display unit 9. According to this method, the position of the buried object in the direction depthwise is arithmetically determined on the basis of the aforementioned time difference and the velocity of propagation of radio wave in the ground. Obviously, the propagation velocity of radio wave varies in dependence on the types of media through which the radio wave propagate. Accordingly, when the velocity of propagation in concern can not be definitely determined through a single cycle of measurement, the time difference is measured for a plurality of times by changing the position of the antenna, whereby the velocity of propagation is determined on the basis of data resulted from the plural measurement. On the other hand, for determining the position of the buried object in the horizontal direction, the antenna 2 is moved to measure the intensity of the echo (reflected radio wave) at various discrete positions of the antenna to determine a horizontal distribution of the echo intensity. Then, the horizontal position of the buried object can be estimated on the basis of the position at which the intensity of echo signal is at maximum.
For the buried or underground object to be detected, there may be mentioned, for example, water-service pipes, gas-supply pipes and others. In this connection, it is noted that the underground pipe or tube is not always made of steel but may be made of other material such as vinyl chloride in dependence on the intended applications. Under the circumstance, in order to determine the position of a particular buried pipe or tube such as a gas-supply pipe, it is indispensably important to detect the material constituting the pipe. Further, the discriminative identification of the objects reflecting the radio wave is important in view of the fact that there may exist under the ground not only the buried pipes such as gas-supply pipe but also rocks, water of pit produced due to leakage of damaged water-service pipes, cavity due to gas leakage, archaeological remains or vestiges, dangerous objects such as unexploded shells and other. In order to identify discriminatively only the aimed buried target object among the various the other buried objects such as those mentioned above, it is necessary to identify discriminatively both the shape or form and the material of the target object.
Heretofore, in conjunction with the identification of the object reflecting the radio wave, there has been adopted a method of examining or checking the difference in the intensity of echo. The concept of this method is based on the fact that, in case that objects in concern are made of plastic (or air) and a metal, respectively and they are located at the same position and have a same shape or configuration, the difference in the intensity of echoes from these objects are about 20 dB. This method can thus be adopted for identifying discriminatively the material of buried objects having a same outer diameter or contour, but suffers a drawback that the method can not be adopted in case the objects in concern differ in respect to the outer diameter and/or the shape.
The buried pipe detecting apparatus known heretofore is capable of displaying the results of the underground investigation in terms of visual information. In that case, when clutter echoes produced by underground rocks and/or cavities are simultaneously displayed as the visual information, the displayed picture will become very complex, to make it difficult to distinguish the requisite information from the unwanted information because the former is covered by the latter. Accordingly, it is also important for the visual display to eliminate the unwanted signals (hereinafter also referred to as the clutter noise) from the reflected wave, to thereby allow only the useful information to be extracted for the processing.